VENTURI / Technical insight
What are Kv and Cv? Understanding flow coefficients
Quick answer
Kv expresses water flow capacity in m³/h at a 1 bar pressure drop under defined conditions. US Cv is approximately 1.156 × Kv. Pipe or connection diameter alone does not determine the required flow capacity.
The difference between Kv, Kvs and Cv
Kv describes capacity at a given opening; Kvs usually denotes the rated Kv at full opening. Do not mix US and Imperial gallon definitions of Cv. The approximate 1.156 multiplier here refers to US Cv. Maximum catalogue capacity does not guarantee good control at every operating point.
A simple water example
For turbulent, incompressible, non-choked flow, a basic estimate is Kv = Q / √(Δp / SG), with Q in m³/h, Δp in bar and SG relative to water. For Q=10 m³/h, Δp=1 bar and SG≈1, Kv≈10. This educational example excludes viscosity, piping reducers, cavitation and control-range corrections.
Which data does control-valve sizing need?
Provide minimum, normal and maximum flows with corresponding inlet/outlet pressures, temperature, density, vapour pressure, pipe sizes and connections. Do not use the liquid formula for gas or steam. Final sizing uses manufacturer coefficients and the applicable calculation method.
Frequently asked questions
How do Kv and US Cv convert?+
Kv expresses water flow capacity in m³/h at a 1 bar pressure drop under defined conditions. US Cv is approximately 1.156 × Kv. Pipe or connection diameter alone does not determine the required flow capacity.
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